This invention pertains generally to the field of magnetic refrigeration and to active magnetic regenerative refrigeration apparatus.
Active magnetic regeneration combines a regenerator with a device which operates on the magnetocaloric effect. The operation of active magnetic regenerators is described in U.S. Pat. No. 4,332,135 to Barclay, et al. An experimental model of an active magnetic regenerator has been built and tested and is described in an article by A. J. DeGregoria, et al., xe2x80x9cTest Results of An Active Magnetic Regenerative Refrigerator,xe2x80x9d Advances in Cryogenic Engineering, Vol. 37B, 1991. A detailed model of the active magnetic regenerator is given in an article by A. J. DeGregoria, Advances in Cryogenic Engineering, Vol. 37B, 1991. An active magnetic regenerator is a type of cooler or heat pump that utilizes the magnetocaloric effect. Materials that exhibit the magnetocaloric effect warm upon magnetization and cool upon demagnetization. In a basic active magnetic regenerator (AMR) device, a bed of magnetocaloric material which is porous to a heat transfer fluid is connected to two heat exchangers, with a mechanism provided for effecting reciprocating fluid flow through the bed of magnetocaloric material from one heat exchanger to the other. A mechanism is also provided for magnetizing and demagnetizing the bed. There are four parts to an AMR cycle: bed magnetization, which warms the magnetocaloric material and the fluid in the bed by the magnetocaloric effect; cold side to hot side fluid flow through the bed with release of heat through a hot side heat exchanger; bed demagnetization, cooling the magnetocaloric material and the fluid in the bed; and hot side to cold side fluid flow through the bed, with the cooled fluid absorbing heat at the cold side heat exchanger.
An AMR device magnetizes and warms the bed prior to fluid flow from cold to hot, and then demagnetizes and cools the bed prior to flow from the hot side to the cold side. The application of the magnetic field to the magnetized bed creates a pair of profiles of temperature and relative position in the bed, one when the bed is magnetized and the other when the bed is demagnetized. The difference between the two bed profiles at any location is the adiabatic temperature change of the magnetocaloric material in going through the change in magnetic field. If the adiabatic temperature change is large enough, the fluid emerging from the cold side of the bed can have a temperature which is lower than the temperature of the cold reservoir, resulting in net cooling of the cold reservoir, rather than a heat leak from the hot reservoir to the cold reservoir which would be the case with an ordinary regenerator. Of course, in accordance with the laws of thermodynamics, work must be done in such a process since heat is flowing from a cold to a hot reservoir. In the case of an AMR, the work is performed by the drive mechanism which moves the magnet and/or the bed relative to one another or by an electrically switched magnet. By utilizing the heat exchangers at both the hot side and the cold side, heat can be removed from the cold side heat exchanger through the AMR and released through the hot side heat exchanger. A structure for accomplishing this transfer is disclosed in the aforesaid U.S. Pat. No. 4,332,135.
A further extension of active magnetic regenerators is shown in U.S. Pat. No. 5,249,424 to DeGregoria, et al., in which the flow of heat transfer fluid through the bed is unbalanced so that more fluid flows through the bed from the hot side to the cold side of the bed than from the cold side to the hot side. The excess heat transfer fluid is diverted back to the hot side of the bed, and multiple stages of active magnetic regenerators may be used. As described in this patent, the regenerator beds may be moved in and out of the magnetic field either in a reciprocating fashion or the beds may be mounted in a rotating wheel.
One of the disadvantages of active magnetic regenerators is the inefficiency encountered because the heat transfer fluid in reciprocating active magnetic regenerators is shuttled back and forth between the regenerator bed(s) and the respective hot and cold heat exchangers. Because the flow of fluid is not in a single direction between the beds and the heat exchangers, some amount of the heat transfer fluid is always in the connecting lines between the beds and the heat exchangers and never cycles both through the beds and the heat exchangers. This trapped heat transfer fluid, commonly referred to as the xe2x80x9cdead volume,xe2x80x9d is a significant source of inefficiency in previous active magnetic regenerators. U.S. Pat. No. 5,934,078 to Lawton, Jr., et al. discloses a reciprocating active magnetic regenerator refrigeration apparatus that greatly reduces the dead volume of heat transfer fluid.
In accordance with the present invention, a rotating bed magnetic refrigeration apparatus has magnetic regenerator beds arranged in a ring that is mounted for rotation about a central axis, such that each bed moves into and out of a magnetic field provided by a magnet as the ring rotates. Each bed has a hot end and a cold end. Heat transfer fluid is directed to and from the regenerator beds by a distribution valve which is connected by conduits to the hot and cold ends of the beds and which rotates with the ring of beds. The distribution valve has a stationary valve member which is connected by conduits to a hot heat exchanger and to a cold heat exchanger. A pump connected in the conduits drives the heat transfer fluid in circulation via the conduits through the hot and cold heat exchangers, the distribution valve and the magnetic regenerator beds. Each of the beds includes magnetocaloric material that is porous and that allows the heat transfer fluid to flow therethrough. The distribution valve directs heat transfer fluid to the hot end of a bed that is outside of the magnetic field such that the heat transfer fluid flows circumferentially through the bed to its cold end where it is directed back to the distribution valve. When a bed is in the magnetic field, the distribution valve directs fluid to the cold end of the bed for flow therethrough circumferentially to the hot end, where the fluid is directed back to the distribution valve, completing an active magnetic regenerator cycle. During each complete revolution of the ring of regenerator beds, the fluid flowing through each conduit flows only in a single direction or remains stationary during a portion of the cycle, minimizing dead volume in the conduits and thereby enhancing efficiency.
A distribution valve may be utilized that comprises an inner stationary valve member and an outer rotating valve member that is mounted to rotate about a central axis in engagement with the stationary valve member. In a preferred rotary distribution valve, the stationary valve member has two cold fluid chambers and two hot fluid chambers, and the rotating valve member has first cold fluid ports therein that, as the rotating valve member rotates, are successively in communication with the first cold fluid chamber, and second cold fluid ports therein that, as the rotating valve member rotates, are successively in communication with the second cold fluid chamber. The rotating valve member further includes first hot fluid ports that, as the rotating valve member rotates, are successively in communication with the first hot fluid chamber, and second hot fluid ports that, as the rotating valve member rotates, are successively in communication with the second hot fluid chamber. Channels are formed in the stationary valve member extending from two hot fluid openings to the first and second hot fluid chambers, and from two cold fluid openings to the first and second cold fluid chambers. Conduits then extend from cold input ports of the beds at the cold ends thereof to the ports in the rotating valve member that come successively in communication with the first of the cold fluid chambers. Conduits also extend from the cold output ports of the beds to the ports in the rotating valve member that come successively in communication with the second of the cold fluid chambers. Conduits also extend from the hot output ports of the beds at the hot ends of the beds to the ports in the rotating valve member that come successively in communication with the first of the hot fluid chambers as well as conduits that extend from the hot input ports of the beds to the ports of the rotating valve member that come successively in communication with the second of the hot fluid chambers. The rotating valve member is connected by the conduits to the beds in the ring of beds and rotates with the ring. Thus, all of the switching of the fluid flow occurs at the central rotary distribution valve rather than at valves engaged with the ring. The seals required for the central rotary distribution valve are efficient and much simpler than seals that would required to engage with the ports at the beds, allowing simplified seal design, reduced wear on the seals, and minimized mechanical losses in the distribution valve.
The distribution valve may also be formed with two disks having flat faces that are tightly engaged to each other. One of the disks is a stationary valve member and the other is a rotating valve member mounted for rotation. The two disks have ports therein that come successively into and out of communication to direct fluid flow to appropriate conduits extending from the rotating disk to the hot end and cold end of each magnetic refrigerator bed. The distribution valve stationary disk is connected by conduits to the hot heat exchanger and cold heat exchanger, and fluid flow is distributed by the distribution valve disks in the same manner as discussed above for the distribution valve having a stationary inner valve member and a rotating outer valve member.
Because the ring can be driven at constant speed in circular motion, greater mechanical efficiency can be obtained than in reciprocating systems. Furthermore, inertial effects can be minimized by reducing the mass of the rotating components. Preferably, the multiple beds forming the ring are arranged with the hot ends of adjacent beds adjacent to one another, and the cold ends of adjacent beds adjacent to one another, to minimize the temperature differences between adjacent beds and therefore minimize thermal leakage between beds. Preferably, the hot ends of adjacent beds are separated by a flow-proof separator. Separators may also be used at the cold ends of adjacent beds but are not necessary, and in a preferred design the cold ends of adjacent beds are open to and in communication with each other.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.